Getting the green light

An international team of scientists has determined the structure of the chlorophyll molecules in green bacteria that are responsible for harvesting light energy.



The team’s results could one day be used to build artificial photosynthetic systems such as those that convert solar energy to electrical energy.


The scientists found that the chlorophylls are highly efficient at harvesting light energy.



‘We found that the orientation of the chlorophyll molecules make green bacteria extremely efficient at harvesting light,’ said Donald Bryant, professor of biotechnology at Penn State University in the US and one of the team leaders.



According to Bryant, green bacteria are a group of organisms that generally live in extremely low-light environments, such as in light-deprived regions of hot springs and at depths of 100m in the Black Sea. The bacteria contain structures called chlorosomes, which contain up to 250,000 chlorophylls.



‘The ability to capture light energy and rapidly deliver it to where it needs to go is essential to these bacteria, some of which see only a few photons of light per chlorophyll per day,’ he said.



Because they have been so difficult to study, the chlorosomes in green bacteria are the last class of light-harvesting complexes to be characterised structurally by scientists. Scientists typically characterise molecular structures using X-ray crystallography, a technique that determines the arrangement of atoms in a molecule and ultimately gives information that can be used to create a picture of the molecule.



However, X-ray crystallography could not be used to characterise the chlorosomes in green bacteria because the technique only works for molecules that are uniform in size, shape and structure.



‘Each chlorosome in a green bacterium has a unique organisation,’ said Bryant. ‘They are like little andouille sausages. When you take cross-sections of andouille sausages, you see different patterns of meat and fat, no two sausages are alike in size or content, although there is some structure inside, nevertheless. Chlorosomes in green bacteria are like these sausages, and the variability in their compositions had prevented scientists from using X-ray crystallography to characterise the internal structure.’



To get around this problem, the team used a combination of techniques to study the chlorosome. The group first used genetic techniques to create a mutant bacterium with a more regular internal structure. Then they used cryo-electron microscopy to identify the larger distance constraints for the chlorosome and solid-state nuclear magnetic resonance (NMR) spectroscopy to determine the structure of the chlorosome’s component chlorophyll molecules. The scientists finally brought all the pieces together and created a computer model to show a complete picture of the chlorosome.



Bryant said that the team’s results may one day be used to build artificial photosynthetic systems that convert solar energy to electricity. ‘The interactions that lead to the assembly of the chlorophylls in chlorosomes are rather simple, so they are good models for artificial systems,’ he added.



‘You can make structures out of these chlorophylls in solution just by having the right solution conditions. In fact, people have done this for many years; however, they haven’t really understood the biological rules for building larger structures. I won’t say that we completely understand the rules yet, but at least we know what two of the structures are now and how they relate to the biological system as a whole, which is a huge advance.’


The team also included researchers from the Leiden Institute of Chemistry and the Groningen Biomolecular Sciences and Biotechnology Institute in the Netherlands and the Max Planck Institute in Germany.